I recently sent my 25+ year old H20-10X (early noise canceling model) and H20-10 to DC for overhaul. They both had been sitting in the closet for nearly two decades.
The 20-10X was overhauled for $300 and the H20-10 for $130. They are as good as new and the H20-10X is actually better than new given that the old multi-AA battery pack was replaced with the newer single 9V battery pack.
I have no idea if any of the other internal components have been improved over the years but other than the faded green plastic shells, they were just about completely rebuilt.